The developing trend of the DC/DC converter is just like that of the most of the power supply products—high efficiency, high power density and low cost. The resonant converters such as the LLC converters are more and more broadly applied to the DC/DC converters due to its advantages of zero-voltage turn-on of the switches on the primary side and the zero-current turn-on/off of the switches on the secondary side at a fall range of load.
However, in the realistic applications, the over-current protection (OCP) is a more critical problem. When a resonant circuit is over-loaded or short-circuited, the resonant current of the circuit is quite large. And if it is not limited, the converter circuit would be damaged or failed due to the large current. OCP circuit could accomplish a rapid protection of the resonant converter under abnormal conditions e.g. over-loaded or short-circuited circumstances, and limit the inrush current during start-up.
In order to engage the OCP towards the resonant circuits, the first method is increasing the operating frequency of the converter. Through increasing the operating frequency of the converter, the impedance of the resonant tank is increased to realize the current limiting. This method is simple and easy to be realized, but has the following drawbacks: under the OCP, the frequency of switches of circuit is higher than the frequency of those under the normal operation, the losses of the switches will increase dramatically; the losses of magnetic elements are increased accordingly; the heat dissipating requirements are raised; the stress on the magnetic elements are increased; and the sizes of elements are also greatly increased.
The second method is a frequency modulation (FM) plus PWM method. In this method, the frequency of the switches is increased firstly; the PWM control circuit begins to operate to modulate the pulse width of the switches when the frequency is increased to a certain extent, thus the voltage added on the resonant tank is decreased to realize the current limiting. This method is relatively more complex. The ZVS would be lost under the current limiting mode, thus the losses of the switches are increased. And the requirements towards the driving circuits are quite rigorous, and at the same time, a rapid protection is required.
The third method realizes the current limiting by clamping the resonant capacitor voltage to the input voltage. FIG. 1 and FIG. 5 are respectively showing the symmetrical half-bridge and the symmetrical full-bridge resonant converters employing this method. In FIG. 1, the half-bridge resonant converter receives a DC input voltage Vin and comprises a square wave generator—the bridge arm formed by the series connected switches Qa and Qb, the resonant inductor Ls, the magnetized inductor Lm, the clamping diode D1-D2, the resonant capacitors Cr1 and Cr2 (Cr1 and Cr2 could have substantially the same capacitance), the transformer T, the rectifying diodes Ds1-Ds2, the output capacitor Cf and the load (the load of the LLC resonant circuit appears to have the characteristics of a current source, and a current source Io is used here to express). In FIG. 5, the full-bridge resonant converter comprises a square wave generator—two bridge arms formed by the four switches Qa, Qb, Qc and Qd, two resonant inductors Ls1 and Ls2 connected in series (e.g. Ls1 and Ls2 could be symmetrical, that is, Ls1 and Ls2 have substantially the same inductance), two magnetizing inductors Lm1 and Lm2 connected in series (e.g. Lm1 and Lm2 could be symmetrical, that is, Lm1 and Lm2 have the same inductance). And Lm1 and Lm2 are the two magnetizing inductors of the transformer T which can also be shown as two inductors connected in parallel with the primary windings of the transformer T as in FIG. 1, e.g., it could be half of the inductance of a single magnetizing inductor Lm, i e Lm/2), four clamping diodes D1 -D4, and a resonant capacitor Cr which is connected to D1 and D2 and to D3 and D4. And the elements of the secondary side of the converter are the same as those in FIG. 1. This method is a passive control method, which is simple and easy to be realized, and does not need extra control circuit to realize the current limiting of each period. In this method, only some clamping diodes and using a symmetrical structure for the resonant circuit are enough. But due to that the clamping voltage is the input DC voltage in this method, the maximum voltage on the resonant capacitor could only be the input voltage. Thus the voltage of the resonant capacitor changes according to of the input voltage, and that is to say the clamping voltage is varied following the change of the input voltage. That will make the design of the resonant circuit to be limited to a certain extent, and the operational scope of the resonant circuit is also influenced.
The fourth method adds an auxiliary transformer to clamp the resonant capacitor voltage. As shown in FIG. 2, it is a symmetrical half-bridge resonant circuit adding the auxiliary transformer T1 to commonly clamp the resonant capacitor Cr voltage to be limited to the secondary side output voltage V0. The voltage on the resonant capacitor Cr is clamped to the output voltage value Vo of the resonant converter through the auxiliary transformer T1 and the clamping diodes D1-D2. Through adding the auxiliary transformer T1, the clamping voltage value could be changed, and the more flexible design for the clamping voltage could be realized. This method could make the design of resonant converter no longer restricted by the clamp voltage to some extent, but an extra magnetic element—a transformer T1 is required. Thus extra loss is generated; the efficiency is decreased; the cost is increased; and there are existing safety and regulation problems too.
The fifth method adds auxiliary winding coupled to the transformer or the resonant inductor and clamping diodes to clamp the resonant capacitor voltage.
As shown in FIG. 3, two half-bridge resonant circuits comprise switches Qa-Qd and resonant inductors Ls1-Ls2 (e.g. Ls1 and Ls2 could have the same inductance value) in their primary sides, and the rectifying diodes Ds1-Ds4 and the output capacitor Cf in their secondary sides, wherein each resonant circuit respectively has the clamping windings Lm12 and Lm22 and the diodes D1-D2 and D3-D4 to clamp the resonant capacitor voltages of Cr1 -Cr2 and Cr3-Cr4 to the input voltage Vin. FIG. 4 shows two half-bridge resonant circuits with the clamping windings CW1 and CW2 coupled to the resonant inductor Ls1 and Ls2 respectively and diodes to clamp the resonant capacitor voltage to the input voltage. The difference between FIG. 4 and FIG. 3 are that the resonant inductors Ls1-Ls2 on the primary side coupled to the clamping windings CW1 and CW2 respectively, while the secondary side of FIG. 4 is the same as that of FIG. 3. This method causes the magnetic elements to have complex configurations, and the losses and the costs to be increased, and the efficiency to be decreased.
In general, one could observe that the above-mentioned related arts either have a complex control, or require extra magnetic elements, and both of which result in the increase of the cost and the volume, and the decrease of the efficiency. The method in FIG. 1 or FIG. 5 is relatively simple, but since the clamping voltage could only be the input voltage, thus the clamping voltage varies with the fluctuation of the DC input voltage, and the operation scope of the converter is limited. Especially during the dynamic switching of the load the DC input voltage of the converter decreases, so does the clamping voltage. Then the clamp circuit is easy to work and the design of the circuit parameters is limited.
Keeping the drawbacks of the prior arts in mind, and employing experiments and research full-heartily and persistently, the applicants finally conceived resonant converter having an over-current protection apparatus and a controlling method thereof.